Carboxylic acids, such as the four carbon succinic, malic, maleic and fumaric acids, as well as their derivatives play an important role as precursor molecules for a variety of other chemicals, including the biodegradable polyester resins, dyestuffs, and pharmaceuticals and as additives in the food industry. Currently, for example, succinic acid is largely produced commercially from crude oil by catalytic hydrogenation of maleic anhydride to succinic anhydride and subsequent hydration or by direct catalytic hydrogenation of maleic acid. This traditional way of producing succinic acid from petrochemicals is costly and causes pollution problems. In recent years, many have sought to develop a more cost competitive and environmentally-friendly way of producing succinic acid by means of a biological-based fermentative process. The fermentative production of an important dicarboxylic acid is advantageous not only because renewable substrates are used, but also because the greenhouse gas CO2 is incorporated into succinic acid during fermentation.
For instance, these biologically-derived succinic acid (BDSA) processes seek to produce succinic acid by fermenting glucose from biomass, separating and purifying the acid, and then catalytically processing it as a platform chemical to produce, for example, 1,4-butanediol (BDO) and related products, tetrahydrofuran and γ-butyrolactone; N-methyl pyrrolidinone (NMP), 2-pyrrolidinone or other chemicals that are used to make a wide assortment of products. Existing domestic markets for such chemicals total almost 1 billion pounds, or more than $1.3 Billion, each year. The BDSA processes also promise to reduce reliance on imported oil and to expand markets for domestic agriculture to more than food sources.
Ordinarily, however, the recovery of dicarboxylic acids from a fermentation broth involves forming insoluble salts of the diacids, typically, insoluble calcium salts. In the case of fermentation by fungi such as Rhizopus oryzae or Asperigillus oryzae, which preferentially make fumaric and malic acid, respectively, the calcium is typically introduced into the broth in the form of CaCO3, which forms Ca(HCO3)2 in solution. The bicarbonate is effective to maintain the pH of the broth as the diacid being produced tends to lower the pH. The diacid is recovered as the calcium salt form. The calcium salts of such C4 diacids have a very low solubility in aqueous solutions (typically less than 3 g/liter at room temperature), and are not suitable for many applications for which the free acid is needed, such as chemical conversion to derivative products like butanediol and the like. Therefore, the calcium salt is typically dissolved in sulfuric acid, forming insoluble calcium sulfate, which can readily be separated from the free diacid. Calcium sulfate is a product having few commercial applications, and accordingly is typically discarded as a solid waste in landfills or other solid waste disposal sites.
In an alternative process described for example in WO2010/147920, instead of using calcium carbonate, the pH of the medium for fungi growth was maintained using a magnesium oxygen containing compound, such as MgO, Mg(OH)2, MgCO3, or Mg(HCO3)2, all of which form the bicarbonate salt in aqueous solution. The use of magnesium rather than calcium was found to enhance production of the acid by fermentation. The fermentation was conducted at a pH of 5-8 and more preferably 6.0-7.0. The pH was maintained by the addition of the magnesium oxygen compound, and CO2 was introduced into the medium in combination with the magnesium oxygen compound to maintain a molar fraction of bicarbonate (HCO3−) of at least 0.1 and most preferably about 0.3 based on the total moles of HCO3−, CO3−2 and CO2 in the medium. At the end of the fermentation, the liquid portion of the medium contained a majority of diacid as a soluble magnesium salt, which was separated from a solids portion of the medium containing precipitated salts and other insoluble material. The dissolved acid salt was converted into the free acid form by reducing the pH to below the isoelectric point of the diacid using a mineral acid such as sulfuric acid, and lowering the temperature of the medium to (most preferably) not greater than 5° C., which precipitated the free acid from the solution.
While useful for producing a free acid, the techniques described for using the magnesium salts results are expensive, first because the magnesium oxygen compounds cost considerably more than the analogous calcium compounds but also because the bulk of the magnesium remains in the fermentation medium in the form of the magnesium salt of the inorganic acid, and is not useful for further fermentation or other purposes. Further, the need to lower the temperature of the recovered soluble salts to precipitate the free acid adds additional energy costs.
Although the fermentative production of carboxylic acids, such as malic or succinic acid, has several advantages over petrochemical-based processes, the generation of carboxylic acid salts as just discussed carries significant processing costs because of the difficulties associated with the downstream processing and separation of the acids and their salts. When salts are generated in conventional fermentation processes, an equivalent of base is required for every equivalent of acid to neutralize. The amount of reagent used can increase costs. Further, one needs to remove the counter ions of the salts so as to yield free acids, and one needs to remove and dispose of any resulting waste and by-products. All of these individual operational units contribute to the overall costs of the process.
Recovery of carboxylic acids as salts has a number of associated problems and requires several different steps in post-fermentation, downstream processing to isolate free acids and to prepare the carboxylic acids for chemical transformation and to convert the raw acids to useful compounds. When salts are generated in conventional fermentation processes, an equivalent of base is required for every equivalent of acid to neutralize. The amount of reagent used can increase costs. Further, one needs to remove the counter ions of the salts so as to yield free acids, and one needs to remove and dispose of any resulting waste and by-products. For instance, calcium salts of C4 diacids have a very low solubility in aqueous broth solutions (typically less than 3 g/liter at room temperature), and are not suitable for many applications for which a free acid species is needed, such as chemical conversion to derivative products. Therefore, the calcium salt is typically dissolved in sulfuric acid, forming insoluble calcium sulfate, which can readily be separated from the free diacid. Calcium sulfate is a product having few commercial applications, and accordingly is typically discarded as a solid waste in landfills or other solid waste disposal sites. All of these individual operational units contribute to the overall costs of the process.
The production costs for the bio-based carboxylic acids have as a result been too high for bio-based production to be cost-competitive with petrochemical production regimes. (See e.g., Janes McKinlay er al., “Prospects for a Bio-based Succinate Industry,” APPL. MICROBIOL. BIOTECHNOL., (2007) 76:727-740; incorporated herein by reference.) For example, with most commercially viable succinate producing microorganisms described in the literature, one needs to neutralize the fermentation broth to maintain an appropriate pH for maximum growth, conversion and productivity. Typically, the pH of the fermentation broth is maintained at or near a pH of 7 by introduction of ammonium hydroxide or other base into the broth, thereby converting the di-acid into the corresponding di-acid salt. About 60% of the total production costs are generated by downstream processing, e.g., the isolation and purification of the product in the fermentation broth.
Over the years, various other approaches have been proposed to isolate the di-acids. These techniques have involved using ultra-filtration, precipitation with calcium hydroxide or ammonia, electrodialysis, liquid-liquid extraction, sorption and ion exchange chromatography. (See, Tanja Kurzrock et al., “Recovery of Succinic Acid from Fermentation Broth,” Review, BIOTECHNOLOGY LETTER, (2010) 32:331-339; incorporated herein by reference.) Alternative approaches that some have proposed include operating a fermentation reactor at low pH, which functionally would be similar to operating the fermentation with minimum level of salts. (See, e.g., Carol A. Roa Engel er al., “Development of a Low-pH Fermentation Strategy for Fumaric Acid Production by Rhizopus oryzae,” ENZYME AND MICROBIAL TECHNOLOGY, Vol. 48, Issue 1, pp. 39-47, 5 Jan. 2011, incorporated herein by reference.)
For example, FIG. 1 shows a schematic diagram of a known process for extracting organic acids from a fermentation broth. Glucose, corn steep liquor, or other sugars, and CaCO3 are introduced into a fermentation reactor 1 and subjected to microbial fermentation 2. A fermentation broth liquid containing a mixture of organic acids and other by-products 3 is extracted and filtered 4. The broth is neutralized 5 with a strong acid, such as H2SO4, which generates CaSO4. The reaction mixture is then filtered 6 to remove cell mass and the CaSO4 7, which is waste that cannot be recycled; hence, it is disposed of in landfill or employed for gypsum-using applications. The remaining organic acids, glycerol, and other by-products 8 can be recovered and fed back into the fermentation reactor as a carbon source, such as described in U.S. Pat. No. 8,183,022, the content of which is incorporated herein by reference. The products can be separated by various techniques, such as crystallization or ion exchange 9. The organic acids can be purified 10, for example, over a carbon bed.
An alternative approach some have described involves the synthesis of alkyl monoesters by direct esterification of alkali metal salts of carboxylic acids, such as calcium lactate, sodium acetate, sodium benzoate, and sodium salicylate, using carbon dioxide and an alcohol as a way of making bio-based chemicals in an environmentally friendly manner (see, Prashant P. Barve, et al., “Preparation of Pure Methyl Esters From Corresponding Alkali Metal Salt of Carboxylic Acids Using Carbon Dioxide and Methanol” IND. ENG. CHEM. RES., 15 Sep. 2011.). The esterification process, however, has a limited application and do not describe the recovery of polycarboxylic acids.
Although these techniques have had some success, they are not able to provide a direct route by which fermentation-derived dicarboxylic or polycarboxylic acids can be recovered in a simple, cost-efficient process from a fermentation broth. Rather, these fermentation techniques often involve the need to go through several different steps to prepare the carboxylic acids in fermentation broth for chemical transformation and to convert the raw acids to useful compounds.
To reduce waste and costs associated with generating free carboxylic acids and to improve the recovery yield, a need exists for a better, more direct method of recovering a variety of carboxylic acids, such as malic or succinic acid, and which can provide a successful route to simplify downstream chemical conversions from a biologically-derived feedstock. Such a streamlined, green process would be a welcome innovation.